Transcript classes/2014/electronics/Lecture 2

Lecture 2
Introduction to Electronics
Rabie A. Ramadan
[email protected]
http://www.rabieramadan.org/classes/2014/electronics/
Signal Amplification
•
Introduce the most fundamental signal processing function employed
in every electronic circuit
Signal Amplification
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Signal Amplification
•
•
•
Transducers produce signals within a range of microvolt or millivolt
signals are too small for reliable processing
processing is much easier if the signal magnitude is made larger.
Detecting
wirelessly
signature of
a virus is
impossible
3
Signal Amplification
• Care must be exercised in the amplification of a signal, so
that the information contained in the signal is not changed
and no new information is introduced.
• when we feed the signal to an amplifier, we want the output
signal of the amplifier to be an exact replica of that at the
input, except of course for having larger magnitude.
• Any change in waveform is considered to be distortion and
is obviously undesirable.
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Signal Amplification
•
An amplifier that preserves the details of the signal
waveform is characterized by the relationship
• where Vi, and Vo are the input and output Signals,
respectively, and A is a constant representing the magnitude
of amplification, known as amplifier gain.
• a linear relationship, hence the amplifier it describes is a
linear amplifier.
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Signal Amplification
• The amplifiers discussed so far are primarily
intended to operate on very small input signals.
• Their purpose is to make the signal magnitude larger
and therefore are thought of as voltage amplifiers.
• The preamplifier in the home stereo system is an
example of a voltage amplifier.
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power amplifier
• power amplifier : an amplifier may provide only a
modest amount of voltage gain but substantial
current gain.
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Amplifier Circuit Symbol
• This common terminal is used as a reference point
and is called the circuit ground
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Voltage Gain
• A linear amplifier accepts an input signal Vi(t)and provides at the output,
across a load resistance RL an output signal V0(t)
• magnified replica of Vi(t)
• The Voltage gain Av of the amplifier is defined by:
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Power Gain and Current Gain
• the Signal power. an Important feature that distinguishes an
amplifier from a Transformer.
• Transformer : although the voltage delivered to the load
could be greater than the voltage feeding the input side (the
primary):
– the power delivered to the load (from the secondary side of the
Transformer) is less than or at most equal to the power supplied by
the Signal. source.
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Power Gain and Current Gain
• On the other hand. an amplifier provides the load With power
greater than that obtained from the signal source.
• The power gain of the amplifier:
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Semiconductors
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What you will learn
• The basic properties of semiconductors and in particular silicon which is
the material used to make most of today's electronic circuits. '
• How doping a pure silicon crystal dramatically changes its electrical
conductivity, which is the fundamental idea underlying the use of
semiconductors In the implementation of electronic devices.
•
• The two mechanisms by which current flows in semiconductors: drift and
diffusion of charge carriers.
• The structure and operation of the pn Junction; a basic semiconductor
structure that Implements the diode and plays a dominant role in
transistors.
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Semiconductors
• The most significant property of semiconductors is
that:
– their conductivity can be varied over a very wide range
through the introduction of controlled amounts of
impurity atoms into the semiconductor crystal in a
process called doping.
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Categories of Solids
• There are three categories of solids, based
on their conducting properties:
–conductors
–semiconductors
–insulators
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Electrical Resistivity
and Conductivity of
Selected Materials
at 293 Kelvin
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Reviewing the previous table reveals that:
• The electrical conductivity at room
temperature is quite different for each of these
three kinds of solids
– Metals and alloys have the highest
conductivities
– followed by semiconductors
– and then by insulators
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Intrinsic Semiconductors
• Semiconductors are materials whose conductivity lies
between that of conductors, such as copper , and insulators
such as glass
• There are two kinds of semiconductors:
– single-element semiconductors, such as germanium and
Silicon
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Intrinsic Semiconductors
• Compound semiconductors are useful in special
electronic Circuit applications as well as in
applications that involve light, such as Iightemitting diodes (LEDs).
• Of the two elemental semiconductors, germanium
was used in the fabrication of very early transistors
(late 1940s, early 1950s).
• It was quickly supplanted, however, with silicon, on
which today’s integrated-circuit (IC) technology is
almost entirely based.
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Atomic Structure
• Atoms go around the nucleolus in their orbits –
discrete distances
• Each orbit has some energy level
• The closer the orbit to the nucleus the less energy it
has
• Group of orbits called shell
• Electrons on the same shell have similar energy level
• Valence shell is the outmost shell
• Valence shell has valence electrons ready to be
freed
• Number of electrons (Ne) on each shell (n)
Ne = 2n2
– First shell has 2 electrons
– Second shell has 8 electrons (not shown here)
Semiconductors
• Remember the further away from the nucleus
the less energy is required to free the electrons
• Germanium is less stable
– Less energy is required to make the
electron to jump to the conduction band
• When atoms combine to form a solid, they
arrange themselves in a symmetrical patterns
• Semiconductor atoms (silicon) form crystals
• Intrinsic crystals have no impurities
Intrinsic Semiconductors
•
•
A silicon atom has four valence electrons, and
thus it requires another four to complete its
outermost shell.
This is achieved by sharing one of its valence
electrons with each of its four neighboring atoms
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Electronic Materials
•
•
The goal of electronic materials is to generate
and control the flow of an electrical current.
Electronic materials include:
1. Conductors: have low resistance which allows
electrical current flow
2. Insulators: have high resistance which suppresses
electrical current flow
3. Semiconductors: can allow or suppress electrical
current flow
A presentation of eSyst.org
Conductors
• Good conductors have low resistance so
electrons flow through them with ease.
• Best element conductors include:
– Copper, silver, gold, aluminum, & nickel
• Alloys are also good conductors:
– Brass & steel
• Good conductors can also be liquid:
– Salt water
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Conductor Atomic Structure
• The atomic structure of good
conductors usually includes
only one electron in their
outer shell.
– It is called a valence electron.
– It is easily striped from the
atom, producing current flow.
Copper
Atom
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Insulators
• Insulators have a high resistance so current does
not flow in them.
• Good insulators include:
– Glass, ceramic, plastics, & wood
• Most insulators are compounds of several
elements.
• The atoms are tightly bound to one another so
electrons are difficult to strip away for current
flow.
A presentation of eSyst.org
Semiconductors
• Semiconductors are materials that essentially can
be conditioned to act as good conductors, or good
insulators, or any thing in between.
• Common elements such as carbon, silicon, and
germanium are semiconductors.
• Silicon is the best and most widely used
semiconductor.
A presentation of eSyst.org
Semiconductor Valence Orbit
• The main characteristic
of a semiconductor
element is that it has
four electrons in its
outer or valence orbit.
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Crystal Lattice Structure
• The unique capability of
semiconductor atoms is
their ability to link
together to form a
physical structure called a
crystal lattice.
• The atoms link together
with one another sharing
their outer electrons.
• These links are called
covalent bonds.
2D Crystal Lattice
Structure
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3D Crystal Lattice Structure
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Semiconductors can be
Insulators
• If the material is pure semiconductor material like silicon, the
crystal lattice structure forms an excellent insulator since all
the atoms are bound to one another and are not free for
current flow.
• Good insulating semiconductor material is referred to as
intrinsic.
• Since the outer valence electrons of each atom are tightly
bound together with one another, the electrons are difficult
to dislodge for current flow.
• Silicon in this form is a great insulator.
• Semiconductor material is often used as an insulator.
A presentation of eSyst.org
Doping
• To make the semiconductor conduct electricity,
other atoms called impurities must be added.
• “Impurities” are different elements.
• This process is called doping.
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Semiconductors can be Conductors
• An impurity, or element
like arsenic, has 5
valence electrons.
• Adding arsenic (doping)
will allow four of the
arsenic valence electrons
to bond with the
neighboring silicon
atoms.
• The one electron left
over for each arsenic
atom becomes available
to conduct current flow.
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Resistance Effects of Doping
• If you use lots of arsenic atoms for doping, there
will be lots of extra electrons so the resistance of
the material will be low and current will flow
freely.
• If you use only a few boron atoms, there will be
fewer free electrons so the resistance will be high
and less current will flow.
• By controlling the doping amount, virtually any
resistance can be achieved.
A presentation of eSyst.org
Another Way to Dope
• You can also dope a
semiconductor material with an
atom such as boron that has only
3 valence electrons.
• The 3 electrons in the outer orbit
do form covalent bonds with its
neighboring semiconductor atoms
as before. But one electron is
missing from the bond.
• This place where a fourth electron
should be is referred to as a hole.
• The hole assumes a positive
charge so it can attract electrons
from some other source.
• Holes become a type of current
carrier like the electron to support
current flow.
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Types of Semiconductor
Materials
• The silicon doped with extra electrons is called
an “N type” semiconductor.
– “N” is for negative, which is the charge of an
electron.
• Silicon doped with material missing electrons
that produce locations called holes is called “P
type” semiconductor.
– “P” is for positive, which is the charge of a hole.
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Current Flow in N-type Semiconductors
• The DC voltage source has a
positive terminal that attracts
the free electrons in the
semiconductor and pulls them
away from their atoms leaving
the atoms charged positively.
• Electrons from the negative
terminal of the supply enter the
semiconductor material and are
attracted by the positive charge
of the atoms missing one of
their electrons.
• Current (electrons) flows from
the positive terminal to the
negative terminal.
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Current Flow in P-type Semiconductors
• Electrons from the negative
supply terminal are attracted to
the positive holes and fill them.
• The positive terminal of the
supply pulls the electrons from
the holes leaving the holes to
attract more electrons.
• Current (electrons) flows from
the negative terminal to the
positive terminal.
• Inside the semiconductor
current flow is actually by the
movement of the holes from
positive to negative.
A presentation of eSyst.org
In Summary
• In its pure state, semiconductor material is an excellent insulator.
• The commonly used semiconductor material is silicon.
• Semiconductor materials can be doped with other atoms to add
or subtract electrons.
• An N-type semiconductor material has extra electrons.
• A P-type semiconductor material has a shortage of electrons with
vacancies called holes.
• The heavier the doping, the greater the conductivity or the lower
the resistance.
• By controlling the doping of silicon the semiconductor material
can be made as conductive as desired.
A presentation of eSyst.org
Recombination Process
• Thermal generation results in free electrons and holes in equal
numbers and hence equal concentrations, where concentration
refers to the number of charge carriers per unit volume(cm 3).
• The free electrons and holes move randomly through the silicon
crystal structure, and in the process some electrons may fill
some of the holes.
• This process, called recombination
disappearance of free electrons and holes.
,results
in
the
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Recombination Process
• In thermal equilibrium, the recombination rate is equal
to the generation rate, and one can conclude that the
concentration of free electrons n is equal to the
concentration of holes p.
• where denotes the number of free electrons and holes in
3
a
unit
volume
(cm
)
of
intrinsic
silicon at a given temperature.
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Recombination Process
•
Results from semiconductor physics gives ni a
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Example
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hole and free- electron
concentration
•
Finally, it is useful for future purposes to express
the product of the hole and free- electron
concentration as :
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Doped Semiconductors
• Concentrations are far too small for silicon to conduct
appreciable current at room temperature.
• Also, the carrier concentrations and hence the conductivity
are strong functions of temperature,
• Not a desirable property in an electronic device.
• Fortunately, a method was developed to change the carrier
concentration in a semiconductor crystal substantially and in
a precisely controlled manner.
• This process is known as doping, and the resulting silicon is
referred to as doped silicon
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Doped Semiconductors
• Doping involves introducing impurity atoms into the silicon
crystal in sufficient numbers to substantially increase the
concentration of either free electrons or holes but with little or
no change in the crystal properties of silicon.
• To increase the concentration of free electrons, n , silicon is
doped with an element with a valence of 5, such as phosphorus.
– The resulting doped silicon is then said to be of n type
• . To increase the concentration of holes, p , silicon is doped
with an element having a valence of 3, such as boron,
– the resulting doped silicon is said to be of p type
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n Type
• A silicon crystal doped with phosphorus impurity. The dopant
(phosphorus) atoms replace some of the silicon atoms in
the crystal structure.
• Since the phosphorus atom has five electrons in its outer shell, four of
these electrons form covalent bonds with the neighboring atoms, and the
fifth electron becomes a free electron.
• Thus each phosphorus atom donates a free electron to the silicon crystal,
and the phosphorus impurity is called a donor
• . It should be clear, though, that no holes are generated by this process.
• The positive charge associated with the phosphorus atom is a bound
charge that does not move through the crystal
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n Type
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P Type
• To obtain p -type silicon in which holes are the
majority charge carriers, a trivalent impurity such as
boron is used.
• a silicon crystal doped with boron.
• Note that the boron atoms replace some of the silicon
atoms in the silicon crystal structure.
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P type
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